Compounds that induce cancer cells to differentiate to a less malignant phenotype provide an attractive area for the development of new anticancer drugs. One would expect differentiating agents to exhibit reduced toxicity relative to conventional chemotherapeutic agents, since the mechanism of antitumor action of differentiating agents is not based primarily on cytotoxicity.
There are presently a number of compounds which are known to influence cell differentiation and growth characteristics. These materials, which include simple organic molecules as well as proteins, are thought to influence gene expression.
A number of cell lines have been found to differentiate in the presence of small molecules. It was originally discovered that a virus-induced murine erythroleukemia cell line (MELC), when treated with dimethyl sulfoxide, expressed many of the features common to terminally differentiated erythroid cells. Another important in vitro differentiation system is the human HL-60 myeloid leukemia cell line. The two cell lines currently in use primarily for evaluating differentiating properties of small molecules are the MELC and the HL-60.
With dimethyl sulfoxide as a lead compound, many organic compounds with varying degrees of effectiveness have been studied as differentiation inducers. Dimethyl formamide (DMF) and N-methylacetamide cause MELC and HL-60 cells to differentiate, but with optimum concentrations of around 150 and 50 mM, respectively, they are not sufficiently potent to be clinically practical if similar concentrations are required in vivo.
Marks et al., in Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 1003-1006; J. Biol. Chem. 1978, 253, 4214-4218; and Biochim. Biophys. Acta 1980, 605, 325-346; reported that placing two amide functions in the same molecule increased compound potency. Both activity and potency are maximized in the polymethylenebis[acetamide] series with five or six methylene groups. More recently, studies with dicarboxylic acid amides (Hozumi et al., Int. J. Cancer 1979, 23, 119-122) and diamine analogues with different acyl groups (Matsuo et al., Acta Haematol. Jpn. 1984, 47, 926-937) have been reported, but none of the compounds tested appeared to be superior to hexamethylene bis(acetamide), HMBA.
HMBA is presently undergoing clinical trials based on differentiation as a mechanism of antitumor action. In vitro studies in the MELC system show that five day exposure to 5 mM HMBA gives optimum results. However, studies with rats and dogs indicate that these conditions might be difficult to maintain in vivo without significant toxicity.